Multibranch pipeline project auth

## Issue

### Context
- **Gitlab plugin version**: 1.5.2
- **Gitlab version**: 10.0.3
- **Jenkins version**: 2.97
- **Job type**: Multibranch pipeline

### Problem description

I have Role-Based authorization Strategy at my jenkins server and users without special permissions can't start any job.
I'm install gitlab plugin; at jenkins configuration check `Enable authentication for '/project' end-point`.
I'm not install any access tokens or something else, just install plugin and set `enable authentication`.
On gitlab project I create push-webhook to jenkins multibranch pipeline project, like `http://jenkins_url:8080/project/test_webhooks/` without secret token.

Then I press `Test` button and recieve `Hook executed successfully: HTTP 200`, but expected `Error 403`

Also I check it for freestyle project. If I not set secret token at job and webhook settings, after `Test` I recieve:
```

Error 403 anonymous is missing the Job/Build permission

HTTP ERROR 403

Problem accessing /project/test_webhooks/. Reason:

    anonymous is missing the Job/Build permission

```

If I set secret token at jenkins freestyle project and not set at webhook settings:
```

Error 401 Invalid token

HTTP ERROR 401

Problem accessing /project/test_webhooks/. Reason:

    Invalid token

```

So, how can I set authorization for multibranch pipeline projects like on freestyle projects?